This invention relates generally to air curtains, and particularly to a vertical air curtain producing a forward or outward ramping effect which may be used with access ways such as drive-through service windows or enclosures such as protective hoods for user-accessible electronic equipment or laboratory instruments.
Various air curtain devices are known to the art, especially for use in situations where a person requires unobstructed access to a workpiece that is kept in a sterile or contaminant free environment, and where a glove box would not be suitable. Conversely, air curtains can work to prevent the escape of chemical vapors, contaminants, or biologically active materials from an enclosure into the surrounding atmosphere, such as in negative pressure hoods. There are several representative examples of conventional air curtain devices.
U.S. Pat. No. 3,327,935 to Berlant discloses a conventional vertical air curtain which permits variable air intake and output volumes.
U.S. Pat. No. 3,408,914 to Bayern discloses a fume hood for a work bench which diverts air out through the front access port when the port is open, and through an exhaust system when the port is closed. U.S. Pat. No. 3,301,167 to Howard discloses a similar fume hood in which the air is recirculated through a sub-micron filter and blown forwardly and downwardly in a linear direction at an angle relative to the front access port and floor plate. Air which does not pass through the access port is drawn back through the perforated floor plate by the blower, and recirculated to the filter.
U.S. Pat. No. 3,356,006 to Scott discloses a clean room structure in which recirculated air is blown through a centrally located arcuate sub-micron filter. The air expelled from the arcuate filter travels linearly in an outward radial pattern to create a continuum of progressively decreasing ambient air pressure surrounding the arcuate filter. Consequently, since the ambient air pressure is greatest directly beneath the arcuate filter, air will travel away from that zone and towards the perimeter of the clean room. While air may be recirculated through vents in the floor and conduits in the walls of the clean room, the clean room may also have one or more open walls.
While suitable for complete or partial enclosures in which a positive pressure environment may be maintained within the enclosure, these devices are not effective when the enclosure itself provides a negative pressure environment, or needs to be separately ventilated. Furthermore, these devices do not mitigate against ambient air being drawn from the exterior of the enclosure into the current of recirculating air.
Also, while the types of air curtains and devices discussed above operate with both full and partial enclosures, they become ineffective in screen- or tunnel-like access ways. To any extent that they might be operational in a screen- or tunnel-like access way, they would function as a source of undirected positive pressure rather than a vertical air curtain. Consequently, while a device such as Scott '006 or Howard '167 could be modified to operate in a tunnel-like system, the tunnel would need to be relatively long, have distinct opposing ends with outwardly directed air flow, and the pressure would decrease rather than increase with proximity to the access ports. In addition, any intermediate area of the tunnel would need to maintain a balancing positive pressure environment, and would restrict ventilation of the intermediate area.
For their most effective operation, conventional air curtain devices rely on two interrelated concepts: creating a generally vertical stream of air, and creating a pressure differential on opposing sides of that stream of air with the pressure differential oriented to assist the air curtain in either excluding air (and particles or contaminants) from the controlled environment or retaining air (and contaminants or fumes) within the controlled environment. These air curtains are not effective, however, where the pressure differential between the controlled environment and the surrounding atmosphere is oriented in direct opposition to the proper functioning of the air curtain (i.e., where the desire is to exclude outside air from a negative pressure controlled environment, or retaining air within a positive pressure controlled environment.)
One particular example relates to drive-through service windows found in "fast-food" type restaurants. In these restaurants, the cooking is generally performed using large frying, grilling, or broiling surfaces which produce a great deal of smoke and spattered grease, and require a significant amount of ventilation. The blowers used to produce this ventilation create strong low pressures within the restaurant which tend to draw air, rain or snow, bugs, exhaust fumes, and oter wind-blown debris through the drive-through windows when the doors thereto are open. Restaurant employees must frequently stand by these drive-through windows for long periods of time, and are therefore exposed to those fumes and objects being drawn forcibly through the windows. In such an instance, the vertical air curtain having an outward ramping effect of this invention has proven very effective in preventing the ingress of fumes, bugs, rain and snow, and many wind-blown objects when the air curtain is mounted across the accessway of the drive-through window, while still permitting continuous access by employees working at the window.
A second example relates to precision equipment or electronic instruments found in extremely dusty environments or those contaminated by airbourne particlate materials, including grain terminals and plants where containers are filled with de-aerated powders such as titanium dioxide Many types of mechanical and electronic instruments (such as weigh scales or the central processing units controlling automated filling lines) can be very susceptible to damage by dust or other fine particulate materials which accumulate on the instruments or are drawn into their housings by cooling fans. Furthermore, heat and electrical sparks within some equipment can pose a serious threat of dust-explosions in certain very contaminated environments. However, it is also often necessary for portions of the instruments to remain readily accessible to workers operating the equipment or using the instruments in those environments. In such instances, it has proven effective to place the instrument or equipment within a partial box-like enclosure having an open front accessway, with the vertical air curtain of this invention mounted to produce an outward ramping effect across that accessway. As such, workers can gain immediate and frequent access to an instrument without it being exposed to the contaminated environment, and while still permitting the instrument to be properly ventilated if necessary.